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1. Introduction

The central nervous system is composed of neuronal and glial cells. Glial cells are broadly classified as astrocytes, oligodendrocytes, or microglia. Microglia are the resident immunocompetent cells in the brain that become rapidly activated in response to neurological impairment during brain injury or disease. Upon activation, microglia alter their numbers and morphology drastically, migrate to the site of insult, phagocytize dead cells, and perform various other functions including inflammatory cytokines release. As a result, microglia have been believed to be involved in the various neurodegenerative disorders of the central nervous system. Past studies have been dedicated to the role of activated microglia in the pathogenesis of brain diseases and damage based on the remarkable changes in morphology and function upon activation. However, recent studies describe an unexpected role of microglia in the healthy brain as determined by recent developments in in vivo imaging techniques. Using high-resolution imaging, the resting microglia were observed to move the branch protuberances frequently and to contact with neuronal synapses 1, 2). Furthermore, synapse numbers are properly maintained in the development process of the cerebral cortex, which suggested that microglia could participate in the control of synapse numbers by the excessive formation and subsequent removal of synapses through contact with synapse directly 3, 4). In this review, we discuss recent findings of the role of microglia for mental disorders. In addition, we introduce some examples of immunohistochemical staining using the new labeled Iba1 antibody which is a particular marker of microglia.

2. Microglia and Mental Disorders

In recent years, there has been a rapid increase in the number of patients with mental disorders. Early identification, and development of methods for early diagnosis and treatment for mental disorders are urgently required because these patients often experience difficulty living in society. Abnormal neural circuit formation and failure of neuronal function maintenance mechanisms during brain development are thought to represent causes of developmental disorders such as autism spectrum disorder (ASD) and schizophrenia. The synapse numbers in primates, including humans, rapidly increases during the neonatal period and peaks during childhood. Excess synapses are "pruned" during adolescence as the brain matures. It has been reported that poor synapse pruning leads to excess new synapses formation in autism patients, whereas excessive synapse pruning occurs in schizophrenia patients, suggesting that abnormal synaptic rate fluctuation patterns during postnatal development are involved in the pathogenesis. For instance, increased microglia and expression of inflammation-related genes in the brains of patients with autism and schizophrenia were found (Fig.1) 5-8). Additionally, it has been proposed that microglia may mediate the development of these diseases through regulation of neural circuit formation and neuronal function maintenance because patient symptoms were improved through minocycline treatment, which suppresses the activation of microglia 9,10).
Our research group has been using the common marmoset, a small primate. Because the common marmosets live in families, various behaviors such as shared offspring rearing responsibilities, as well as social behaviors and communication can be observed; they represent a class of useful experimental animals for understanding the social brain functions of humans. Moreover, drug metabolism pathways, physiological and anatomical features, and expressed genes are similar to those in humans 11). Fluctuation patterns in synapse numbers during cerebral cortex development are also similar to those in humans. Therefore, the marmoset may be a useful experimental animal to understand functional neural circuit formation in primates. We created an autism-like marmoset model by exposing marmoset embryos to valproic acid 12,13). The use of this marmoset model to understand the role of microglia in the pathology of autism spectrum disorder may lead to a better understanding of autism and other mental disorders and the development of novel treatment methods.

Fig. 1: Increase in active microglia in the brains of patients with autism spectrum disorder (ASD)Positron emission tomography (PET) utilizing radiotracer [11C]PK11195 that specifically binds to activated microglia revealed that active microglia are increased in the brains of patients with autism 8).

—From rodents to primates—
Iba1 (Ionize calcium binding adapter molecule 1) is an approximately 17-kDa calcium-binding protein found in the central nervous system, and it is specifically expressed in microglia 14). The Iba1 antibody is widely used as a microglial marker, and Iba1 antibody product is labeled with biotin or a red fluorescent dye which have recently become commercially available. We used mouse and rat brain slice specimens to perform immunohistochemical staining using a biotin-labeled and red fluorescence-labeled Iba1 antibody (Fig.2). Results confirmed that microglia, and even their narrow projections were stained using labeled antibodies. Furthermore, we found that biotin-labeled antibodies can be used in marmoset brain-slice specimens. Non-specific staining levels were lower in brain slice specimens of all species than in those when a secondary antibody was used. Additionally, time and steps for staining were also less than those for conventional staining with the secondary antibody. Therefore, expansion of its use can be expected.

Fig. 2: Immunohistochemical staining using labeled Iba1 antibodies
Microglial staining was confirmed using labeled antibodies even within narrow projections
in the cerebral cortex of mouse, rat and marmoset.

4. Conclusion

Although the role of activated microglia has been noted in human diseases, recent developments in imaging techniques have led to the finding that microglia are actively involved in the maintenance and regulation of healthy brain function. The new labeled Iba1 antibodies introduced in this paper may contribute to the expansion of microglial research. Microglia have great potential that remains to be revealed, and it is expected that new functional roles of glial cells will be elucidated.

7. Note

Common marmoset (Callithrix jacchus): small primate that is 20-30 cm in length. They have the highest reproductive efficiency among primates,
reach sexual maturity at 18 months after birth, have a pregnancy duration of approximately 150 days, and give birth twice a year. Two offspring per birth is common,
but marmosets in captivity may have three or four offspring per birth. In 2009, a genetically modified marmoset model was created, the first such model in primates
following this, various disease marmoset models have been actively developed.
Synapse: the structure of junctions formed between nerve cells. Synaptic involvement in neural activity such as signaling and formation of neural circuits
by neurons is necessary for the expression of normal higher brain functions.